Opaque silica glass article having transparent portion and process for producing same

ABSTRACT

An opaque silica glass article comprising a transparent portion and an opaque portion, wherein the opaque portion has an apparent density of 1.70-2.15 g/cm 3  and contains 5×10 4 -5×10 6  bubbles per cm 3 , said bubbles having an average diameter of 10-100 μm; and the transparent portion has an apparent density of 2.19-2.21 g/cm 3  and the amount of bubbles having a diameter of at least 100 μm in the transparent portion is not more than 1×10 3  per cm 3 .  
     The opaque silica glass article is made by a process wherein a mold is charged with a raw material for forming the opaque portion, which is a mixture comprising a silica powder with a small amount of a silicon nitride powder, and a raw material for forming the transparent portion so that the two raw materials are located in the positions corresponding to the opaque and the transparent portions, respectively, of the silica glass article to be produced; and the raw materials are heated in vacuo to be thereby vitrified.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] This invention relates to an opaque silica glass article and aprocess for producing the same. More particularly, it relates to anopaque silica glass article comprising a transparent portion and anopaque portion, and having good heat insulating property and goodsurface smoothness, and to a process for producing the opaque silicaglass article by melt-forming together a raw material for the opaqueportion and a raw material for the transparent portion into an articleof an arbitrary shape.

[0003] (2) Description of the Related Art

[0004] An opaque silica glass article has good heat-insulating property,i.e., is capable of cutting-off heat rays transferring as radiant heat.In the case where the silica glass article contains a salient amount offine bubbles uniformly distributed therein, its heat-insulatingperformance is superior.

[0005] One example of the opaque silica glass article is a flangeprovided at the base of a furnace tube used as a furnace for heating asilicon wafer, as illustrated in FIG. 1. A heating race illustrated inFIG. 1 has heretofore used widely for heating a silicon wafer, whichcomprises a heating element 1, a furnace tube 2, a boat 4 for supportingsilicon wafers 3, an insulating cylinder 5 and a base 6. A flange 9 isprovided at the base of the furnace tube 2. The flange 9 is made ofopaque silica glass and welded together with the furnace tube 2 by anoxyhydrogen flame. The flange 9 has a function of heat insulation forcutting off heat transferring to the base 6 and a packing 7, which havea poor heat resistance. A desired atmosphere can be kept within thefurnace tube 2 by the seal by means of packing 7 between the flange 9and the base 6. Opaque silica glass is widely used in many fieldsincluding the flange of a heating furnace.

[0006] The opaque silica glass article is usually made by a method forheating a powdery siliceous raw material to melt and vitrify the rawmaterial. The method for heating the raw material includes, for example,Verneuil's method wherein the raw material is subjected to flame fusionby using an argon-oxygen plasma flame or an oxyhydrogen flame, and avacuum melting method wherein a vessel is charged with the raw materialand the raw material is heated and melted in vacuo.

[0007] As the raw material for the opaque silica glass article, naturalsilica rock or stone, and rock crystal of a low quality level haveheretofore been widely used. These raw materials contain a multiplicityof fine bubbles therein, and, when the raw materials are melted forvitrification, the bubbles remain within he glass to yield opaque silicaglass articles.

[0008] In recent years, LSI is being highly integrated in the field of asemiconductor, and thus a raw material with a high purity of an opaquesilica glass article is eagerly desired. A most typical example of thesilica glass article is the above-illustrated flange of a furnace tubeused in a furnace for heating a silicon wafer. However, natural rawmaterials used for the production of an opaque silica glass articlecontain a salient amount of impurities as well as a salient amount offine bubbles, and the bubbles are very difficult to remove. Namely it isdifficult to obtain a raw material with a high purity by purification.On the other hands, a rock crystal with a relatively high puritycontains a minor amount of fine bubbles therein in the crystal, andtherefore, even when the rock crystal is melted, the degree ofopaqueness is not enhanced and the resulting silica glass article istranslucent.

[0009] To solve the above-mentioned problems of the prior art, manyproposals have been made. For example, a process has been proposedwherein an amorphous silica with a high purity which contains reducedamounts of an alkali metal, an alkaline earth metal, iron and aluminum,and a salient amount of fine bubbles, and has a silanol group as avaporizable ingredient contained uniformly at a specific concentrationis subjected to flame fusion (Japanese Unexamined Patent Publication(abbreviated to “JP-A”) H6-24711). However, only silica glass articleshaving a simple shape such as an IC (Integrated circuit)-sealing silicafiller and a matrix ingot for silica glass powder can be directlyproduced, and after-treatments such as after-shaping by lathing arenecessary for the production of silica glass articles with a complicatedshape such as a flange-form, a ring-shape, column, square pillar orhollow-square pillar. Utilization of the raw material is low in theproduction of silica glass articles with a complicated shape, and thus,the production cost is inevitably increased.

[0010] As another process for producing an opaque silica glass article,a process has been proposed wherein a highly purified crystalline silicapowder is heated in an ammonia atmosphere and then the thus-ammoniatedsilica powder is heated and melted in an inert gas atmosphere to give anopaque silica glass article having an increased number of very finebubbles, i.e., having a large total cross-sectional area of bubbles perunit volume of the opaque silica glass, and thus exhibiting an enhancedheat insulation (JP-A H7-61827 and JP-A H7-300341). However, thisprocess has problems such that the density of opaque silica glass, andthe diameter and amount of bubbles contained therein greatly variesdepending upon the particle diameter and particle diameter distributionof raw material powder and the state of raw material powder charged in avessel for fusion, and thus, the diameter and amount of bubbles in thesurface portion and those in the central portion greatly differ fromeach other, and an opaque silica glass article having bubbles uniformlydistributed therein is difficult to produce with good reproducibility.

[0011] As still another process for producing an opaque silica glassarticle, a process has been proposed wherein a finely divided powder ofa foaming agent such as carbon or silicon nitride is incorporated in asiliceous raw material such as silica rock or stone, a-quartz orcristobalite, and the mixture is subjected to a flame fusion using anoxyhydrogen flame (JP-A H4-65328). The abovementioned problems can besolved by this proposed process. However, the use of oxyhydrogen flameinvites introduction of a hydroxyl group within silica glass which leadsto reduction of the viscosity of molten glass and results in an opaquesilica glass article not suitable as articles used for a long period oftime at a high temperature, such as a jig for the production ofsemiconductor devices. Further, in this flame fusion step, the residencetime of finely divided particles in the flame is very short, and thecompletion of reaction in the flame is difficult and it is possible thatthe foaming agent incorporated remains in the molten material as aforeign matter, and fiber that the siliceous raw material reacts withthe forming agent with the result of undesirable coloration of themolten material.

[0012] It is said that, when a silica glass jig for the production of asemiconductor is cleaned after the use thereof, the bubbles exposed onthe surface is removed, i.e., the surface is partly scraped down. Tosolve this problem, a procedure has been adopted for adhering aprotective transparent silica glass film of a predetermined shape on thesurface by heating with oxyhydrogen flame or in an electric furnace.

[0013] For the flange provided at the base of a furnace tube of aheating furnace for a silicon wafer, a heat insulating property as wellas a sealing property are required to stably control the atmospherewithin the furnace tube. Conventional opaque silica glass flanges have arough surface due to the presence of bubbles and thus, even where apacking is used, a complete seal cannot be attained. For overcoming thisdefect, a flange having an opaque portion with good heat insulatingproperty and a transparent portion with good sealing property issuitable.

[0014] Several processes have been proposed for producing the flangehaving an opaque portion with good heat insulating property and atransparent portion with good sealing property is suitable. As examplesof such processes, there can be mentioned (1) a process forfusion-bonding a transparent silica glass article to an opaque silicaglass article, (2) a process wherein a powdery raw material for anopaque silica glass is added to a taparent silica glass article and thecombination thereof is fusion-bonded, (3) a process wherein a powderyraw material for an opaque silica glass and a powdery raw material for ataparent silica glass are melted, and (4) a process wherein a surfaceportion of an opaque silica glass article containing bubbles therein ismelted whereby bubbles within the surface portion is removed and thusthe surface portion is rendered transparent.

[0015] The above-recited processes have the following problems. Namely,in the process of (1), at the step of fusion-bonding, bubbles are liableto occur at the interfacial boundary between the transparent silicaglass portion and the opaque silica glass portion thereof In general theadhesion between the transparent portion and the opaque portion thereofis not sufficient and the adhered transparent portion and opaque portionare liable to be separated. Further when the shape of the opaque silicaglass article is complicated, the transparent silica glass becomes verydifficult to fabricate and to fusion-bond to the opaque silica glass.

[0016] In the process of (2), bubbles do not readily occur at theinterfacial boundary between the two silica glass portions, but thepowdery raw material for the opaque silica glass portion shrinks in thecourse from the fusion bonding step to the completion of vitrification,and thus the resulting silica glass article is liable to warp. Morespecifically, JP-A H7-300326 discloses a process wherein a transparentsilica glass article is placed in a heat-resistant mold, a powdery rawmaterial for forming opaque silica glass is superposed upon thetransparent silica glass article, and then the combined material issubjected to fusion bonding in an inert gas atmosphere to give a silicaglass article having an opaque silica glass layer and a transparentsilica glass layer. In this process, when the superposed powdery rawmaterial containing an inert gas among the particles is melted andvitrified, the inert gas contained among the particles is entrappedwithin the molten material and becomes bubbles in the resulting glassarticle. However, the amount of gas derived from the raw material, thenumber and diameter of bubbles occasionally vary and the bubbles aredifficult to uniformly distribute within the glass, and sometimes aninert gas introduced at the step of fusion bonding becomes part of thebubbles within the glass. Therefore, the bubbles within the opaqueportion of the silica glass article are difficult to control.

[0017] In the process of (3), the gas contained in the powdery rawmaterial for forming an opaque portion partly penetrates into thepowdery raw material for forming a transparent portion with the resultof occurrence of bubbles in the vicinity of the interfacial boundary.Further the opaque silica glass portion and the transparent silica glassportion, both of which shrink in the course from fusion bonding to thecompletion of vitrification, exhibit different shortages, and thus, theresulting silica glass article tends to warp.

[0018] In the process of (4), it is difficult to melt uniformly inthickness the surface portion of the bubble containing opaque silicaglass article, and further to desecrate the molten surface portion to asatisfying extent.

SUMMARY OF THE INVENTION

[0019] In view of the foregoing, a primary object of the presentinvention is to provide an opaque silica glass article having atransparent portion and an opaque portion containing bubbles uniformlydistributed therein, characterized as exhibiting excellenthigh-temperature viscosity and heat insulation, and having a smoothsurface, i.e., not having a roughness, which has occurred due to bubblescontained in the glass article, over the entire surf-ace or part of thesurface.

[0020] Another object of the present invention is to provide a processfor producing the above-mentioned opaque silica glass articleindustrially advantageously, whereby a silica glass article of acomplicated shape such as, for example, a flange-form, ring-shaped,columnar, square pillar or hollow-square pillar can be directly producedfrom raw materials.

[0021] In accordance with the present invention, there is provided anopaque silica glass article comprising a transparent portion and anopaque portion, wherein the glass of the opaque portion has an apparentdensity of 1.70 to 2.15 g/cm³ and contains 5×10⁴ to 5×10⁶ bubbles percm³ of the glass, said bubbles having an average bubble diameter of 10to 100 μm; and the glass of the transparent portion has an apparentdensity of 2.19 to 2.21 g/cm³ and the amount of bubbles having adiameter of at least 100 μm in the transparent portion is not more than1×10³ per cm³ of the glass.

[0022] In another aspect of the present invention, there is provided aprocess for producing the above-mentioned opaque silica glass article,which comprises the steps of:

[0023] charging a heat-resistant mold with a raw material for formingthe opaque portion of the silica glass article, which is a uniformmixture comprising a finely divided silica powder having an averageparticle diameter of 10 to 500 μm with 0.001 to 0.05 parts by weight,based on 100 parts by weight of the silica powder, of a finely dividedsilicon nitride powder, and a raw material for forming the transparentportion of the silica glass article so that the two starting materialsare located in the positions corresponding to the opaque portion and thetransparent portion, respectively, of the silica glass article to beproduced; and

[0024] heating the raw materials in vacuo at a temperature in the rangeof the melting temperature of the raw materials and 1,900° C. wherebythe raw materials are vitrified.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a vertical cross-sectional view illustrating a furnacefor heating silicon wafers;

[0026]FIG. 2 is a perspective view illustrating a heat-resistant moldhaving a ring-form cavity, which is cut along a central vertical plane;

[0027]FIG. 3 is a perspective view of a heat-resistant mold having aflange-shaped cavity, which is cut along a central vertical plane;

[0028]FIG. 4 is a perspective view of a flange-shaped opaque silicaglass article made by using the mold illustrated in FIG. 3;

[0029]FIG. 5 is a perspective view of a heat-resistant mold having acolumnar cavity, which is cut along a central vertical plane;

[0030]FIG. 6 is a perspective view of a columnar opaque silica glassarticle made by using the mold illustrated in FIG. 5;

[0031]FIG. 7 is a perspective view of a ring-form opaque silica glassarticle made by using the mold illustrated in FIG. 2;

[0032]FIG. 8 is a perspective view of a heat-resistant mold having asquare pillar-form cavity, which is cut along a central vertical plane;

[0033]FIG. 9 is a perspective view of a square pillar-form opaque silicaglass article made by using the mold illustrated in FIG. 8;

[0034]FIG. 10 is a perspective view of a heat-resistant mold having ahollow square pillar-form cavity, which is cut along a central verticalplane;

[0035]FIG. 11 is a perspective view of a hollow square pillar-formopaque silica glass article made by using the mold illustrated in FIG.10;

[0036]FIG. 12 is a vertical cross-sectional view illusions aheat-resistant mold in which a powdery raw material is charged;

[0037]FIG. 13 is a perspective view illustrating the powdery rawmaterial-charged heat-resistant mold illustrated in FIG. 12;

[0038]FIG. 14 is a side view of an opaque silica glass article made byusing the mold illustrated in FIG. 12 and FIG. 13;

[0039]FIG. 15 is a perspective view of the opaque silica glass articleillustrated in FIG. 14;

[0040]FIG. 16 is a cross-sectional view of a heat-resistant mold chargedwith a raw material for forming a ring-form transparent silica glassarticle;

[0041]FIG. 17 is a cross-sectional view of a heat-resistant mold chargedwith a ring-form transparent silica glass article and a powdery rawmaterial for forming an opaque portion of a silica glass article;

[0042]FIG. 18 is a perspective view of an opaque silica glass articlemade by using the mold illustrated in FIG. 17, which article is cutalong a central vertical plane; and

[0043]FIG. 19 is a perspective view of an opaque silica glass articlemade by using the mold illustrated in FIG. 17, which is a comparativearticle and is cut along a central vertical plane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] (1) Raw Materials

[0045] As the raw material for forming an opaque portion of the silicaglass article of the invention, a mixture of a finely divided silicapowder and a silicon nitride powder is preferably used As the rawmaterial for forming a transparent portion of the silica glass articleof the invention, a finely divided silica powder or a shaped transparentsilica glass article is used. More specifically, in the opaque silicaglass article having an opaque portion and a transparent portion of theinvention (hereinafter abbreviated to “silica glass article of theinvention”), the opaque portion constituting the main part of the silicaglass article of the invention is made preferably by melting a mixtureof a finely divided silica powder and a finely divided silicon nitridepowder. The transparent portion of the silica glass article of theinvention, which forms the entirety or a part of a surface of the silicaglass article, is made from a finely divided silica powder or a shapedtransparent silica glass article. In the following explanation of thetransparent portion, either a finely divided silica powder or a shapedsilica glass article is used, but it should be construed that any of thesilica powder and the shaped silica glass article can be used

[0046] (1-1) Silica Powder

[0047] As the finely divided silica powder used in the invention, afinely divided crystalline or amorphous silica powder having a highpurity and containing metal impurities such as Na, K, Mg and Fe each inan amount of 0 to 1 ppm is preferably used for the following reason.When the silica glass article of the invention is heated, for example,when a flange made thereof is attached to a wafer-heating furnace and isexposed to a high temperature, the vaporization of impurities exhibitinga high vapor pressure leading to an environmental contamination, thepartial crystallization of the silica glass article of the inventionoccasionally leading to rupture thereof, and the coloration of thesilica glass article of the invention can be avoided by the use of thehigh-purity silica powder.

[0048] The high-purity silica powder is prepared by a synthesis processor a purification of a natural raw material. For example, an amorphoussilica powder is synthesized by a process wherein an aqueous alki metalsilicate solution (water-glass) is reacted with an acid to remove analkali metal thereby yielding silica, a process for hydrolyzing SiCl₄ togive silica, and a process wherein a silicon alkoxide is hydrolyzed togive silica. Of these, the first process, especially a process whereinan aqueous alkali metal silicate solution (water-glass) composed of analkali metal such as Na, K or Li and silicon dioxide is allowed to reactwith an inorganic acid such as sulfuric acid, nitric acid orhydrochloric acid, is preferable from a viewpoint of commercialproduction. A crystalline silica powder can be obtained from natural rawmaterial by treating natural qua with hydrofluoric acid.

[0049] Preferably the finely divided silica powder has an averageparticle diameter of 10 to 500 μm in view of fluidity for charging intoa heat-resistant mold. If the average particle diameter is smaller than10 μm, the silica powder has a poor fluidity and is difficult to chargeinto the mold. In contrast, if the average particle diameter is larger500 μm voids among the particles are too large and large bubbles havinga diameter of at least 300 μm tend to occur, and especially, in the casewhere a transparent portion of the silica glass article is formed fromthe silica powder, a salient amount of large bubbles having a diameterof larger than 500 μm occasionally occur.

[0050] The diameter of bubbles contained in the silica glass article ofthe invention varies depending upon the particular average diameter ofthe silica powder, and thus, the bubble diameter can be varied bycontrolling the average diameter of the silica powder. Namely, finebubbles with a small diameter and bubbles with a large diameter can beobtained from a silica powder having a small average diameter and asilica powder having a large average diameter, respectively.

[0051] (1-2) Silicon Nitride Powder

[0052] As the silicon nitride powder, a high-purity silicon nitrideprepared by nitriding a starting material such as silicon tetrachloride,silicon or silica is preferably used.

[0053] By using the high-purity silicon nitride powder, when theresulting opaque silica glass article of the invention is heated, thevaporization of impurities exhibiting a high vapor pressure leading toan environmental contamination, the partial crystallization of thesilica glass article of the invention occasionally leading to rupturethereof, and the coloration of the silica glass article of the inventioncan be avoided.

[0054] The amount of the silicon nitride powder is 0.001 to 0.05 partsby weight based on 100 parts by weight of the silica powder. If theamount of the silicon nitride powder is smaller than 0.001 parts byweight, the amount of bubbles formed is too small, and the opaque silicaglass article has a poor heat insulation. In contrast, if the amount ofthe silicon nitride powder is larger than 0.05 parts by weight, thebubbles formed become too large and the opaque silica glass article hasa poor mechanical strength.

[0055] The silicon nitride powder preferably has an average particlediameter of 0.1 to 1 μm, more preferably 0.1 to 0.5 μm. By using thesilicon nitride powder having an average particle diameter fallingwithin this range, the amount and size of bubbles formed becomeadequate, and the uniform mixing of the silicon nitride powder and thesilica powder can be effected without agglomeration.

[0056] (2) Mixing

[0057] A finely divided silica powder and a finely divided siliconnitride powder are mixed together to prepare a raw material for formingthe opaque portion of the silica glass article of the invention. Theextent and state of dispersion of the silicon nitride powder in themixture influences upon the diameter and distribution of bubbles formed,the silicon nitride should be uniformly dispersed in the powderymixture. The mixing means is not particularly limited provided that auniform dispersion can be obtained. For example, a mortar and a ballmill can be used. To obtain a highly uniform dispersion of the siliconnitride powder in the powdery mixture, a wet process using a dispersingmedium is preferably employed. As examples of the dispersing medium,there can be mentioned water and alcohols such as ethanol and methanol.To enhance the dispersibility of the silicon nitride powder in thepowdery mixture, an ultrasonic vibration can be applied by using anultrasonic generator.

[0058] (3) Charging of Raw Material in Mold

[0059] The raw material for forming the transparent portion of thesilica glass article and the raw material for forming the opaque portionthereof are charged in a heat-resistant mold.

[0060] First, charging of a finely divided silica powder as thetransparent portion-forming raw material will be explained.

[0061] Namely, the silica powder as the transparent portion-forming rawmaterial and the silicalsilicon nitride powdery mixture as the opaqueportion-forming raw material are placed in a heat-resistant mold. Thematerial and shape of the heat-resistant mold are not particularlylimited provided that the mold exhibits a good resistance and does notinfluence the raw material at the melt fusion step. The heat-resistantmold may be either a single mold or a split mold composed of two or moreparts. The split mold is used for molding a silica glass article havinga complicated shape. The shape and combination of two or more parts ofthe split mold can be appropriately chosen depending upon the desiredshape of the opaque silica glass article. As the material of the mold,there can be mentioned those which do not react with silica to anyappreciable extent, such as carbon, boron nitride and silicon carbide.To impart a good sliding property between the inner wall of the mold andthe raw material, carbon felt or carbon paper can be inserted betweenthe inner wall of the mold and the raw material during charging of theraw material and heating the raw material.

[0062] The raw material for forming the opaque portion of the silicaglass article (namely, a powdery silica/silicon nitride mixture) and theraw material for forming the transparent portion thereof (namely asilica powder) are placed in the mold so that the raw materials arelocated in the positions corresponding to the opaque portion and thetransparent portion, respectively, of the silica glass article to beshaped.

[0063] The shape and size of the heat-resistant mold is determineddepending upon the desired shape and size of the opaque silica glassarticle. For example, when a heat-resistant mold having a flange-shapedcavity as illustrated in FIG. 3 is used, a flange-shaped opaque silicaglass article as illustrated in FIG. 4 is produced. When aheat-resistant mold having a columnar cavity as illustrated in FIG. 5 isused, a columnar opaque silica glass article as illustrated in FIG. 6 isproduced. When a heat-resistant mold having a ring-form cavity asillustrated in FIG. 2 is used, a ring-form opaque silica glass articleas illustrated in FIG. 7 is produced. When a heat-resistant mold havinga polyhedron cavity such as square pillar-form cavity as illustrated inFIG. 8 is used, a polyhedron (such as square pillar-form) opaque silicaglass article as illustrated in FIG. 9 is produced. When aheat-resistant mold having a hollow square pillar-form cavity asillustrated in FIG. 10 is used, a hollow square pillar-form opaquesilica glass article as illustrated in FIG. 11 is produced. As amodified form of the ring-form article of FIG. 7, a ring-form silicaglass article, one end of which is closed, can be produced. Similarly,as a modified form of the hollow square pillar-form article of FIG. 11,a hollow square-form silica glass article, one end of which is closed,can be produced.

[0064] In a specific example of charging the heat-resistant mold withthe raw materials, a silica powder is laid on the bottom of aheat-resistant mold having a columnar cavity, a powdery silica/siliconnitride mixture is laid thereon and further a silica powder is laidthereon. By heating the thus-filled powdery raw materials, a columnaropaque silica glass article having a transparent top layer, an opaquecentral body and a transparent bottom layer is produced. In anotherspecific example of charging the heat-resistant with the raw materials,a silica powder is laid on the bottom of a heat-resistant mold having acolumnar cavity, a cylindrical auxiliary fame having a diameter slightlysmaller than the diameter of the columnar cavity is placed on the laidsilica powder, a powdery silica/silicon nitride mixture is chargedwithin the cylindrical auxiliary frame, a silica powder is filled in acylindrical space between the cylindrical auxiliary frame and the innerwall of the mold, the cylindrical auxiliary frame is drawn out gently;and finally, a silica powder is laid on the top of the charged rawmaterials. By heating the thus-filled raw materials, a columnar opaquesilica glass article having a transparent layer covering the entiresurface of the glass article can be produced.

[0065] The packing density of the powdery raw materials within the moldis preferably in the range of 0.7 to 1.8 g/cm³. The packing density ofthe raw material for forming the opaque portion should preferably be asuniform as possible so as to form the opaque portion having bubblesuniformly dispersed in the opaque portion.

[0066] Secondly, placing of a transparent silica glass article as thetransparent portion-forming raw material within the mold will beexplained.

[0067] In one specific example, the powdery silica/silicon nitridemixture for forming the opaque portion and a transparent ring-formsilica glass article for forming the transparent portion are charged inthe mold having a ring-form cavity as illustrated in FIG. 2. Thetransparent ring-form silica glass article is previously fabricated sothat it is capable of being placed within the mold. The transparentring-form silica glass article preferably has an apparent density of2.19 to 2.21 g/cm³ and contains not more than 1×10³ bubbles per cm³,said bubbles having a diameter of at least 100 μm. An opaque silicaglass article made by using the transparent ring-form silica glassarticle is characterized in that the exposed surface of the transparentportion does not have bubbles to any appreciable extent and thus, whenthe surface is subjected to cleaning, surface roughness due todevelopment of bubbles does not occur, and thus, good sealing propertycan be obtained.

[0068] The shape and size of the transparent ring-form silica glassarticle are not particularly limited provided that it can be placedwithin the mold and it forms a sealing surface of the resulting opaquesilica glass article. Usually the transparent ring-form silica glassarticle has a shape and size corresponding to those of the resultingopaque silica glass article.

[0069] The transparent ring-form silica glass article can be prepared bya process wherein the silica powder is melted by applying an oxyhydrogenflame or melted in an electric furnace in vacuo to give a transparentsilica glass block, followed by grinding the block into the desiredshape and size. In the process employing an electric furnace, preferablya heat-resistant mold having a ring-form cavity having a sizesubstantially the same as that of the transparent portion of the opaquesilica glass article is used. By using this mold, a transparentring-form silica glass article having a size similar to that of thetransparent portion can be produced, and thus, the after-processing ofthe transparent ring-form silica glass article is easy and simple, andman hour and material loss are minimized.

[0070] The material of the heat-resistant mold used is not particularlylimited provided that it is resistant to heat and does not influence theraw material to any appreciable extent at the heating step. For example,the heat-resistant mold can be made of materials which do not easilyreact with silica, such as carbon, boron nitride and silicon carbide.

[0071] To enhance the sliding property of the raw materials on the innerwall of the mold, preferably carbon felt or carbon paper is placedbetween the inner wall of the mold and the raw materials at the step ofcharging and heat-melting.

[0072] The order of charging of the transparent silica glass article andthe powdery silica/silicon nitride mixture for forming the opaqueportion is not particularly limited, but, it is preferable that thetransparent silica glass article is first placed on the bottom of themold, and then the powdery mixture is charged on the silica glassarticle because undesirably large compaction of the powdery mixture canbe avoided and gases evolved at the heat-melting step can be effectivelyremoved. The packing density of the powdery mixture is preferably in therange of 0.7 to 1.8 g/cm³ for uniformly charging it in the mold.

[0073] (4) Vitrification and Bubble Formation

[0074] In order to completely decompose silicon nitride in the powderysilica/silicon nitride mixture to form bubbles and to convert thepowdery silica/silicon nitride mixture into an opaque silica glass, andfiber, to convert a silica powder, if used, as a raw material forforming the transparent portion into a transparent glass, the rawmaterials charged within the mold are heated to be thereby melted. Theheating apparatus used for heating the mold is not particularly limitedprovided that it is capable of converting the raw material into a glassstate, and, for example, an electric furnace is used.

[0075] The raw materials are heated to a temperature between thetemperature at which the raw materials are melted, and 1,900° C. When anamorphous silica powder is used as a raw material, it is melted viacristobalite, and thus, the temperature at which the raw materials aremelted is 1,713° C. at normal pressure. When a crystalline silica powderother than cristobalite is used as a raw material, it is meltedsubstantially without through cristobalite and, thus, the temperature atwhich the raw materials are melted is lower than the above meltingtemperature for the amorphous silica powder. It should be noted that,when a crystalline silica powder other than cristobalite is heated to atemperature lower than the melting temperature, at least part of thisraw material is not melted, the resulting silica glass is very fragile.When an amorphous silica powder is used and a part or the entiretythereof is transferred to crystalline cristobalite, the cristobalite isnot melted at the heating step, and the resulting glass is very fragile.If the raw material is heated to a temperature higher than 1,900° C.,the opaque portion of the opaque silica glass article have bubbles witha large size, and consequently, the density of glass becomes low, andthe mechanical strength is too low to machine the glass article into adesired shape and size. The heating time varies depending upon theparticular heating temperature, and is not particularly limited providedthat the entire amount of the raw material is melted and vitrified.Usually the heating time is about one hour or shorter.

[0076] In the course of heating the raw material, it is preferable thata vacuum atmosphere is kept during a period spanning from the statewherein pores among the particles of the powdery raw material are opento the state wherein said voids are closed. The degree of vacuum ispreferably such that the pressure is not higher than 50 mmHg, morepreferably not higher than 10 mmHg. By conducting the heating in vacuo,gases eluted from nitrogen in the solid solution produced by thereaction of silicon nitride with silica in the powdery silica/siliconnitride mixture, and gases generated by decomposition of the rawmaterial form bubbles uniformly distributed in the silica glass article.Further, when a transparent silica glass article is used as atransparent portion-forming raw material, the residual fine bubbleswithin the transparent portion can be removed.

[0077] In the course where the raw materials charged in a mold aremelted in vacuo whereby they are vitrified and bubbles are formed, acover made of; for example, carbon or the like can be placed on thecharged raw materials so that a uniform pressure is applied onto theentire raw materials, or the bubbles formed are confined within themolten material or controlled so as not to escape to the outside.

[0078] At the time when the molten material maintained at a hightemperature is converted to a glass state, an inert gas is introducedinto a mold. The inert gas used is not particularly limited providedthat it does not react substantially with the mold, the raw material andthe product, and includes, for example, nitrogen, argon and helium. Ofthese, nitrogen and argon are preferable in view of the cost and airtightness. The pressure of the inert gas is usually normal pressure sothat, when the resulting glass is reheated, for example, subjected toflaming treatment, bubbles within the glass are neither greatly expandednor shrunk. A slightly higher or lower pressure may be employed.

[0079] After the heating for vitrification, the molten material iscooled to room temperature. Usually the molten material is cooled byallowing it to stand or by a cooling apparatus to about 1,000° C. Therate of cooling is usually about 1,000° C./hour. Finally the material iscooled to room temperature. It should be noted that, the molten materialtends to crystallize in the course of cooling, especially at a hightemperature. Therefore, the molten material should be cooled relativelyrapidly in a high temperature region to avoid the undesirablecrystallization. To enhance the rate of cooling, the same inert gas asthat used at the heat-melting step can be introduced. In a lowtemperature region in the course of cooling, there is no problem ofcrystallization, and thus, the material is usually left to stand forcooling.

[0080] (5) Opaque Silica Glass Article

[0081] The silica glass article of the invention has an opaque portionhaving an apparent density of 1.70 to 2.15 g/cm³, preferably 1.80 to2.12 g/cm³ and containing 5×10⁴ to 5×10⁶ bubbles per cm³ which bubbleshave an average particle diameter of 10 to 100 μm. These characteristicsare important for imparting good mechanical strength and processabilityto the glass article.

[0082] The diameter and amount of independent bubbles contained in theopaque portion vary depending upon the amount of silicon nitride powderadded, the particle diameter and distribution of silica powder, themelting temperature and the pressure of gas introduced. For example, anopaque portion with good heat insulating property, which has an apparentdensity of 1.95 to 2.05 g/m³, and contains 7×10⁵ to 8×10⁵ bubbles havingan average bubble diameter of 50 to 70 μm, is obtained by selecting thefollowing conditions: amount of silicon nitride powder added=0.01 to0.02 part by weight based on 100 parts by weight of silica powder,average particle diameter of silica powder=100 to 200 μm (particlediameter distribution range=10 to 600 μm), melting temperature of 1,800to 1,850° C., pressure of introduced gas of 1.0 to 2.0 kgf/cm². Anopaque portion with high heat insulating property, which has an apparentdensity of 2.05 to 2.12 g/cm³, and contains 1×10⁶ to 2×10⁶ bubbleshaving an average bubble diameter of 30 to 50 μm, is obtained byselecting the following conditions: amount of silicon nitride powderadded=0.005 to 0.02 part by weight based on 100 parts by weight ofsilica powder, average particle diameter of silica powder=50 to 100 μm(particle diameter distribution range=10 to 200 μm), melting temperatureof 1,750 to 1,850° C., pressure of introduced gas of 1.0 to 2.0 kgf/cm².The amount of bubbles greatly varies depending upon the particlediameter of silica powder. More specifically an opaque silica glassarticle having an excellent heat insulating property, which contains alarge amount of bubbles having a small average diameter, is obtained byusing a finer silica powder.

[0083] The opaque portion of the glass article of the invention containsbubbles uniformly distributed therein and has a white appearance. Thewhite opaque portion is characterized as possessing preferably a lineartransparency of not larger than 5% as measured by irradiating the opaqueportion with light having a wavelength of 300 to 900 nm and expressed asthe value at a thickness of 1 mm. By the reduced linear transparency,heat rays are readily scattered, and thus, the silica glass article ofthe invention exhibits excellent heat insulating property as well as areduced thermal conductivity.

[0084] The transparent portion of the opaque silica glass article, whichhas a function of protecting the surface of the opaque portion, ischaracterized as having an apparent density of 2.19 to 2.21 g/m³. Theamount of bubbles having a diameter of at least 100 μm in thetransparent portion is not more than 1×10³ per cm³ of the glass. If theamount of bubbles with a diameter of at least 100 μm is more than 1×10³per cm³, a salient amount of bubbles are exposed on the surface of thetransparent portion, and good sealing property cannot be obtained.Further, the transparent portion preferably has a linear transparency ofat least 90% as measured by irradiating said portion with thetransparent portion with light having a wavelength of 300 to 900 nm andexpressed as the value at a thickness of 1 mm. When the lineartransparency is at least 90%, the sealing property is more enhanced.

[0085] According to the process of the invention, a hydroxyl group isnot introduced in the glass at the step of heat-fusion, but is ratherexpected to be volatilized from the molten material. The opaque silicaglass article containing the thus-reduced amount of a hydroxyl groupexhibits a high viscosity at a high temperature, i.e., excellenthigh-temperature viscosity.

[0086] The shape of the opaque silica glass article of the invention isnot limited and is suitably chosen depending upon the particular usethereof. For example, the shape thereof is flange-form, ring-shaped,columnar, square pillar or hollow-square pillar.

[0087] Especially, when a ring-form opaque silica glass article is usedfor a flange attached to a furnace tube, the glass article preferablyhas a wall thickness of not larger than 150 mm and a height (i.e., alength along the axis of the ring) of 30 to 250 mm in view of theuniformity in density of the opaque portion thereof and the heatinsulation thereof.

[0088] The ratio of the opaque portion to the transparent portion variesdepending upon the particular use, but the amount of the transparentportion in the opaque silica glass is preferably in the range of 2 to30% based on the sum of the transparent portion and the opaque portion.

[0089] The invention will now be specifically described by the followingexamples that by no means limit the scope of the invention.

[0090] The characteristics of raw materials and opaque silica glassarticles were determined by the following methods.

[0091] (1) Impurity

[0092] The impurities contained in a silica powder were analyzed by ICP(inductively coupled plasma) atomic emission spectrochemical analysis.

[0093] (2) Glass State

[0094] The glass state of the transparent portion and opaque portion ofan opaque silica glass article was examined by X-ray diffraction asfollows.

[0095] A specimen having a size of 20 mm×10 mm×2 mm (thickness) was cutby a cutter from each of the opaque portion and the transparent portion.Each specimen was examined by an X-ray diffraction analyzer (supplied byMAC Science Co., type MXP3), and the glass state was confirmed by thepresence of diffraction peak occurring due to crystals such as quartzand cristobalite in the obtained diffraction pattern.

[0096] (3) Apparent Density

[0097] A specimen having a size of 30 mm×30 mm×10 mm (thickness) was cutby a cutter from each of the opaque portion and the transparent portion.Density of each specimen was measured by using an electronic forcebalance (supplied by Mettler Instrument Co., type AT261) according tothe Archimedean method.

[0098] (4) Diameter and Amount of Bubbles

[0099] A specimen having a size of 30 mm×10 mm×0.3 mm (thickness) wascut by a cutter from each of the opaque portion and the transparentportion. The diameter and amount of bubbles in each specimen weremeasured by using a polarization microscope having a lens withgraduation(supplied by Olympus Optical Co., Ape BH-2) The averagediameter of bubbles in the opaque portion was determined by countingnumber of bubbles, calculating the total volume of the bubbles providedthat the bubbles are regarded as having a spherical form, dividing thetotal volume of bubbles by the number of bubbles to determine theaverage bubble volume, and then, calculating the average diameter, i.e.,the average bubble diameter. The amount of bubbles in the transparentportion was determined by counting the number of bubbles having adiameter of at least 100 μm in a view field of 10 mm×10 mm×0.3 mm(depth) and calculating the number of bubbles per cm³.

[0100] (5) Particle Diameter

[0101] Distribution of particle diameter and average particle diameterof a powdery raw material were measured by the laser diffractionscattering method using Coulter LS-130 (supplied by Coulter ElectronicsCo.)

[0102] (6) Packing Density

[0103] Packing density of a powdery raw material was determined bypacking a predetermined amount of the powdery raw material in aheat-resistant mold, and dividing the amount by weight of the packedmaterial by the volume occupied by the packed material.

[0104] (7) Presence of Pore

[0105] A glass article was cut by a cutter and the presence of pores inthe cut surface was checked by visual examination.

[0106] (8) Light Transmission (Linear Transparency)

[0107] Each of the opaque portion and the transparent portion was cutinto a rectangular plate, and both major surfaces of the paste werepolished by an alumna abrasive grain of #1200 to prepare a specimenhaving a size of 30 mm×10 mm×1 mm (thickness). The linear transparencywas measured by irradiating the specimen with light having a wavelengthof 300, 500, 700 or 900 nm, projected perpendicularly to the majorsurfaces of the specimen (band-pass 2 nm) by using a spectrophotometer(supplied by Hitachi Ltd., double-beam spectrophotometer type 220).

[0108] (9) Total Cross-Sectional Area of Bubbles

[0109] Bubbles are regarded as having a spherical form, and the totalcross-sectional area of bubbles is defined as the sum of circles eachincluding the diameter of bubble. The total cross-sectional area ofbubbles was determined by calculating the average cross-sectional areaof bubbles from the average bubble diameter, and multiplying the averagecross-sectional area of bubbles by the amount of bubbles.

EXAMPLE 1

[0110] Powdery natural quartz having an average particle diameter of 300μm and a particle diameter distribution in the range of 30 to 500 μm wastreated with hydrofluoric acid to prepare a high-purity powdery silica(hereinafter referred to as “powdery quartz”). Silicon tetrachloride wastreated with ammonia to prepare a powdery silicon nitride having anaverage particle diameter of 0.5 μm. A powdery nature of powdery quartzwith the powdery silicon nitride was prepared as follows. 0.01 part byweight of the powdery silicon nitride was put into 50 parts by weight ofethanol, and the mixture was stirred while an ultrasonic vibration wasapplied. To the thus-prepared silicon nitride dispersion, 100 parts byweight of powdery quartz was incorporated and the mixture was thoroughlystirred. Then ethanol was removed from the mixture by using a vacuumevaporator and the mixture was dried to obtain a powdery quartz/siliconnitride mixture (hereinfer referred to “mixed powder”) as a raw materialfor forming an opaque portion of an opaque silica glass article.

[0111] The above-mentioned powdery quartz was also used as a rawmaterial for forming a transparent portion of the opaque silica glassarticle. Namely, as illustrated in FIG. 12 and FIG. 13, 300 g of powderyquartz 12 as the raw material for forming the transparent portion wascharged in a cylindrical carbon crucible 14 having an outer diameter 130mm, an inner diameter of 100 mm and a depth of 200 mm and having carbonfelt 13 with a thickness of 2 mm adhered on the inner wall of thecrucible. 900 g of the mixed powder 11 was placed on the charged powderyquartz 12. The charged powdery quartz 12 and the charged mixed powder 11had a packing density of 1.4 g/cm³.

[0112] The state of the charged powdery quartz 12 and the charged mixedpowder 11 is illustrated in FIG. 12 and FIG. 13. The crucible 14 wasplaced in an electric furnace, and the inner atmosphere was vacuumed toa pressure of 1×10⁻³ mmHg. Then the temperature was elevated from roomtemperature to 1,800° C. at a rate of 300° C./hour. The crucible wasmaintained at 1,800° C. for 10 minutes, and then, a nitrogen gas wasintroduced into the electric furnace until the inner pressure reachednormal pressure (1 kgf/cm²) and the heating was ceased. Thereafter thepower switch of the electric surface was turned out and the crucible wasallowed to stand. The inner temperature of the electric furnace reached1,000° C. about 50 minutes later, and gradually fell to roomtemperature.

[0113] The thus-made glass article was a columnar opaque silica glassarticle having a structure composed of an opaque portion 15 having amultiplicity of bubbles distributed therein, and a transparent portion16 firmly bonded to the opaque portion 15, as illustrated in FIG. 14 andFIG. 15.

EXAMPLE 2

[0114] The same powdery quartz as that used in Example 1 was pulverizedby using a dry ball mill and further sieved to obtain a powdery quartzhaving an average particle diameter of 500 μm and a particle diameterdistribution in the range of 10 to 200 μm. 100 parts by weight of thepowdery quart and 0.03 part by weight of silicon nitride powder wasmixed together to obtain a powdery mixture. By substantially the sameprocedure as that employed in Example 1,300 g of the powdery quartz wascharged in a carbon crucible and then 900 g of the powdery mixture wascharged on the powdery quartz. The charged powdery quartz and thecharged powdery mixture had a packing density of 1.4 g/cm³. The chargedraw materials were heated and then cooled by the same procedure as thatin Example 1 to obtain a columnar opaque silica glass article composedof an opaque portion 15 and a transparent portion 16 firmly bonded tothe opaque portion 15, as illustrated in FIG. 14 and FIG. 15.

EXAMPLE 3

[0115] The same powdery quartz as that used in Example 1 was pulverizedby using a dry ball mill and further sieved to obtain a powdery quartzhaving an average particle diameter of 50 μm and a particle diameterdistribution in the range of 10 to 200 μm. A powdery mixture of thepowdery quartz and a silicon nitride powder was prepared by the sameprocedure as that employed in Example 1. By substantially the sameprocedure as that employed in Example 1,300 g of the powdery quartz wascharged in a carbon crucible and then 900 g of the powdery mixture wascharged on the powdery quartz. The charged powdery quartz and thecharged powdery mixture had a packing density of 1.4 g/cm³. The chargedraw materials were heated and then cooled by the same procedure as thatin Example 1 to obtain a columnar opaque silica glass article composedof an opaque portion 15 and a transparent portion 16 firmly bonded tothe opaque portion 15, as illustrated in FIG. 14 and FIG. 15.

EXAMPLE 4

[0116] The procedures described in Example 1 were repeated to obtain acolumnar opaque silica glass article composed of an opaque portion 15and a transparent portion 16 firmly bonded to the opaque portion 15, asillustrated in FIG. 14 and FIG. 15, wherein the crucible charged withthe powdery quartz and the mixed powder was maintained at 1,850° C.instead of 1,800° C. in the electric furnace with all other conditionsremaining the same. The charged powdery quartz and the charged mixedpowder had a packing density of 1.4 g/cm³ as measured before the chargedpowdery quartz and the charged mixed powder were heated to 1,850° C.

EXAMPLE 5

[0117] The procedures described in Example 1 were repeated to obtain acolumnar opaque silica glass article composed of an opaque portion 15and a transparent portion 16 firmly bonded to the opaque portion 15, asillustrated in FIG. 14 and FIG. 15, wherein, after the crucible chargedwith the powdery quartz and the mixed powder was maintained at 1,800° C.for 10 minutes in the electric furnace, a nitrogen gas was introducedinto the electric furnace until the inner pressure reached 2.0 kgf/cm²and the heating was ceased. All other conditions remained the same. Thecharged powdery quartz and the charged mixed powder had a packingdensity of 1.4 g/cm³ as measured before the charged powdery quartz andthe charged mixed powder were heated to 1,800° C.

EXAMPLE 6

[0118] Powdery amorphous silica having an average particle diameter of300 μm and a particle diameter distribution in the range of 50 to 1,000μm was prepared by a process wherein sodium silicate was reacted with anacid and then the reaction product was heated. The powdery amorphoussilica was pulverized by using a dry ball mill and further sieved toobtain powdery amorphous silica having an average particle diameter of180 μm and a particle diameter distribution in the range of 10 to 600 μmpowdery amorphous silica/silicon nitride mixture was prepared by thesame procedure as that employed in Example 1, from 100 parts by weightof the powdery amorphous silica and 0.01 part by weight of the samepowdery silicon nitride as that used in Example 1 as follows. Namely,300 g of the powdery amorphous silica as a raw material for forming thetransparent portion was charged in the same carbon crucible as that usedin Example 1, and 900 g of the powdery amorphous silica/silicon nitridemixture as a raw material for forming the opaque portion was placed onthe charged powdery amorphous silica. The charged powdery amorphoussilica and the charged amorphous silica/silicon nitride mixture had apacking density of 0.81 g/cm³. The charged materials were heated andthen cooled under the same conditions as those employed in Example 1 toobtain a columnar opaque silica glass article composed of an opaqueportion 15 and a transparent portion 16 firmly bonded to the opaqueportion 15, as illustrated in FIG. 14 and FIG. 15.

EXAMPLE 7

[0119] Powdery amorphous silica having the same average particlediameter and particle diameter distribution as those mentioned inExample 6 was prepared by a process wherein a silicon alkoxide wasreacted with water and then the reaction product was heated. The powderyamorphous silica was pulverized by using a dry ball mill and farthersieved to obtain a powdery amorphous silica having an average particlediameter of 180 μm and a particle diameter distribution in the range of10 to 600 μm. A powdery amorphous silica/silicon nitride mixture wasprepared by the same procedure as that employed in Example 1, from 100parts by weight of the powdery amorphous silica and 0.01 part by weightof the same powdery silicon nitride as that used in Example 1 asfollows. Namely, 300 g of the powdery amorphous silica as a raw materialfor forming the transparent portion was charged in the same carboncrucible as that used in Example 1, and 900 g of the powdery amorphoussilica/silicon nitride mixture as a raw material for forming the opaqueportion was placed on the charged powdery amorphous silica. The chargedpowdery amorphous silica and the charged amorphous silica/siliconnitride mixture had a packing density of 0.81 g/cm³. The chargedmaterials were heated and then cooled under the same conditions as thoseemployed in Example 1 to obtain a columnar opaque silica glass articlecomposed of an opaque portion and a transparent portion firmly bonded tothe opaque portion.

[0120] The X ray diffraction analysis of the opaque silica glassarticles made in Examples 1 to 7 revealed that the opaque portion andthe transparent portion of each of the opaque silica glass articles werein glass state.

[0121] The properties of the opaque silica glass articles made inExamples 1 to 7 were evaluated. Namely, the apparent density, averagebubble diameter and bubble amount of the opaque portion of each glassarticle are shown in Table 1.

[0122] The total cross-sectional area of bubbles and light transmittanceof the opaque portion of each glass article are shown in Table 2. Theapparent density, amount of bubbles with a diameter of at least 100 μm,and light transmittance of the transparent portion are shown in Table 3.TABLE 1 Example Apparent density Average bubble Number of No. (g/cm³)diameter (μm) bubbles per cm³ 1 2.01 74 4 × 10⁵ 2 1.82 88 5 × 10⁵ 3 2.1034 2 × 10⁶ 4 1.86 90 4 × 10⁵ 5 1.97 63 8 × 10⁵ 6 1.96 63 8 × 10⁵ 7 2.0566 5 × 10⁵ 8 2.01 74 4 × 10⁵

[0123] TABLE 2 Total cross- Example sectional area Light transmittance(%) No. of bubbles (cm²/cm³) 300 nm 500 nm 700 nm 900 nm 1 18 0.7 1.32.0 2.7 2 30 0.2 0.4 0.5 0.6 3 20 0.5 1.0 1.6 2.0 4 26 0.3 0.5 0.6 0.8 525 0.4 0.7 1.0 1.4 6 26 0.2 0.3 0.4 0.4 7 16 0.8 1.4 2.3 2.9 8 18 0.71.3 2.0 2.7

[0124] TABLE 3 Example Apparent Number of Light transmittance (%) No.density(g/cm³) bubbles per cm³ 300 nm 500 nm 700 nm 900 nm 1 2.20 50 9295 95 95 2 2.20 50 92 95 95 95 3 2.20 50 92 95 95 95 4 2.20 50 92 95 9595 5 2.20 50 92 95 95 95 6 2.20 50 92 95 95 95 7 2.20 50 92 95 95 95 82.20 50 92 95 95 95

COMPARATIVE EXAMPLE 1

[0125] The same powdery quartz as that used in Example 1 was pulverizedby using a dry ball mill, and for dispersed in ethanol to be sedimentedThus, a powdery quartz having an average particle diameter of 5 μm and aparticle diameter distribution in the range of 1 to 10 μm was obtaineddepending upon the difference in sedimentation rate. A powderysilica/silicon nitride mixture was prepared from the thus-preparedpowdery silica and the same silicon nitride powder as that used inExample 1 by the same procedure as described in Example 1. Bysubstantially the same procedure as that employed in Example 1, 300 g ofthe powdery quart was charged in a carbon crucible and then 900 g of thepowdery silica/silicon nitride mixture was charged on the powderyquartz. The charged powdery quartz and the charged powdery mixture had apacking density of 0.90 /cm³. The charged raw materials were heated andthen cooled by the same procedure as that in Example 1 to obtain acolumnar opaque silica glass article composed of an opaque portion and atransparent portion firmly bonded to the opaque portion.

[0126] The X ray diffraction analysis of the columnar opaque silicaglass article revealed that both the opaque portion and transparentportion thereof were in glass state. However, the opaque portion had alow apparent density, i.e., 1.2 g/cm³, and, when the glass article wascut and the cross-section was visually examined, the glass articleproved to have pores having a diameter of about 2 to 5 mm. Thetransparent portion also has a low apparent density, i.e., 2.15 g/cm³ ₁and proved to have pores having a diameter of about 2 mm.

COMPARATIVE EXAMPLE 2

[0127] The procedures described in Example 1 were repeated to obtain acolumnar opaque silica glass article composed of an opaque portion 15and a transparent portion 16 firmly bonded to the opaque portion 15, asillustrated in FIG. 14 and FIG. 15, wherein the crucible charged withthe powdery quartz and the mixed powder was maintained at 1,950° C.instead of 1,800° C. and the inner pressure of the electric furnace waschanged to 1.0 kg/cm² with all other conditions remaining the same. Thecharged powdery quartz and the charged mixed powder had a packingdensity of 1.4 g/cm³ as measured before the charged powdery quartz andthe charged mixed powder were heated to 1,950° C.

[0128] The X ray diffraction analysis of the columnar opaque silicaglass article revealed that both the opaque portion and transparentportion thereof were in glass state. However, the opaque portion has alow apparent density, i.e., 1.5 g/cm³. The average bubble diameter was200 μm, and the opaque silica glass article was very brittle.

EXAMPLE 8

[0129] A powdery quartz/silicon nitride mixture was prepared by the sameprocedure as mentioned in Example 1 wherein the amount of the powderysilicon nitride was changed to 0.03 part by weight based on 100 parts byweight of the powdery quartz with all other conditions remaining thesame.

[0130] As illustrated in FIG. 16, 5 kg of the same powdery quartz 19 asthat used in Example 1 was charged in a carbon mold 10 with a ring-formcavity having an outer diameter of 440 mm, an inner diameter of 270 mmand a depth of 100 mm and having a carbon felt 18 with a thickness of 5mm adhered on the inner wall of the mold. The state of the chargedpowdery quartz 19 was illustrated in FIG. 16. The mold was placed in anelectric furnace and the inner atmosphere was vacuumed to a pressure of1×10⁼³ mmHg. Then the temperature was elevated from room temperature to1,800° C. at a rate of 300° C./hour. The mold was maintained at 1,800°C. for 10 minutes, and then, the power switch of the electric furnacewas turned out and the mold was allowed to stand. The inner temperatureof the electric furnace reached 1,000° C. about 50 minutes later, andgradually fell to room temperature. The thus-prepared transparentring-form silica glass article was cut to obtain specimens, and theirproperties were evaluated. The apparent density, amount of bubbles witha diameter of at least 100 μm, and light transmittance as irradiatedwith light of wavelength of 300 to 900 nm of the specimens were 2.20g/cm³, 50 bubbles per cm³, and 92 to 95%, respectively. The transparentring-form silica glass article was machined to obtain a transparentring-form silica glass article having an outer diameter of 440 mm, aninner diameter of 270 mm and a thickness (height) of 10 mm, used as araw material for forming the transparent portion.

[0131] As illustrated in FIG. 17, the above-mentioned transparentring-form silica glass article 20 for forming the transparent portionwas placed on the bottom of the same mold 10 as the above-mentionedcarbon mold, which had a carbon felt 18 adhered on the inner wallthereof, and 5 kg of the above-mentioned powdery silica/silicon nitridemixture 21 was charged on the transparent ring-form silica glass article20. The charged powdery silica/silicon nitride mixture 21 had packingdensity of 1.4 g/cm³.

[0132] The raw materials-charged mold was placed in an electricalfurnace, and the inner atmosphere of the furnace was vacuumed to apressure of 1×10⁻³ mmHg. Then the temperature was elevated from roomtemperature to 1,800° C. at a rate of 300° C./hour. The mold wasmaintained at 1,800° C. for 10 minutes, and then, a nitrogen gas wasintroduced into the electric furnace until the inner pressure reachednormal pressure (1 kgf/cm²) and the heating was ceased. Thereafter thepower switch of the electric furnace was turned out and the crucible wasallowed to stand. The inner temperature of the electric furnace reached1,000° C. about 50 minutes later, and gradually fell to roomtemperature.

[0133] As illustrated in FIG. 18, the thus-made glass article was aring-form opaque silica glass article having a structure composed of anopaque ring-form portion 23 and a transparent ring-form portion 22firmly bonded to the opaque portion 23.

[0134] The X ray diffraction analysis of the opaque ring-form silicaglass article revealed that the opaque portion and the transparentportion were in glass state. The properties of the opaque ring-formsilica glass article. Namely, the apparent density, average bubblediameter and bubble amount of the opaque portion of glass article areshown in Table 1. The total cross-sectional area of bubbles and lighttransmittance of the opaque portion of glass article are shown in Table2. The apparent density, amount of bubbles with a diameter of at least100 μm, and light transmittance of the transparent portion are shown inTable 3.

COMPARATIVE EXAMPLE 3

[0135] By the same procedures as employed in Example 8, a powderyquartz/silicon nitride mixture as a raw material for forming the opaqueportion of the opaque silica glass article was prepared wherein powderyquartz having an average particle diameter of 700 μm and a particlediameter distribution in the range of 500 to 1,000 μm was used with allother conditions are the same, and fewer, a transparent ring-form silicaglass article as a raw material for forming the transparent portion ofthe opaque silica glass article was prepared.

[0136] The transparent ring-form silica glass article was placed on thebottom of the same carbon mold as used in Example 8, and 5 kg of thepowdery quartz/silicon nitride mixture was charged on the transparentring-form silica glass article. The charged transparent ring-form silicaglass article had a packing density of 0.78 g/cm³. The rawmaterials-charged mold was placed in an electric furnace, and heated andcooled under the same conditions as employed in Example 8 to obtain anopaque ring-form silica glass article having a structure composed of anopaque portion 23 and a transparent portion 22 firmly bonded to theopaque portion 23, as illustrated in FIG. 19.

[0137] The X ray diffraction analysis of the opaque ring-form silicaglass article revealed that the opaque portion 23 and the transparentportion 22 were in glass state. However, the opaque portion 23 has a lowapparent density, i.e., 1.4 g/cm³, and, when the glass article was cutand the cross-section was visually examined, the glass article proved tohave pores having a diameter of about 0.5 to 1 mm. The transparentportion 22 also has a low apparent density, i.e., 2.17 g/cm³, and provedto have pores having a diameter of about 1 mm.

[0138] The same powdery quartz/silicon nitride mixture and the sametransparent ring-form silica glass article as those prepared in Example8 were prepared. Further, as illustrated in FIG. 19, a multiplicity ofpores 24 having a diameter of about 2 to 3 mm were present in theboundary between the opaque portion 23 and the transparent portion 22.

COMPARATIVE EXAMPLE 4

[0139] The transparent ring-form silica glass article was placed on thebottom of the same carbon mold as used in Example 8, and the powderyquartz/silicon nitride mixture was charged on the transparent ring-formsilica glass article. The charged transparent ring-form silica glassarticle had a packing density of 1.4 g/cm³. The raw materials-chargedmold was placed in an electric furnace, and the inner atmosphere wasvacuumed to a pressure of 1×10⁻³ mmHg. Then a nitrogen gas wasintroduced into the electric furnace until the inner pressure reachednormal pressure (1 kgf/cm²), and then, the temperature was elevated fromroom temperature to 1,800° C. at a rate of 300° C./hour. The mold wasmaintained at 1,800° C. for 10 minutes, and then, the heating wasceased. Thereafter the power switch of the electric furnace was turnedout and the mold was allowed to stand. The inner temperature of theelectric furnace reached 1,000° C. about 50 minutes later, and graduallyfell to room temperature.

[0140] As illustrated in FIG. 19, the thus-made glass article was aring-form opaque silica glass article having a structure composed of anopaque portion 23 and a transparent portion 22 firmly bonded to theopaque portion 23.

[0141] The X ray diffraction analysis of the opaque ring-form silicaglass article revealed that the opaque portion 23 and the transparentportion 22 were in glass state. However, the opaque portion 23 has a lowapparent density, i.e., 1.2 g/cm³, and when the glass article was cutand the cross-section was visually examined, it was found that bubbleswere distributed non-uniformly in the glass article, i.e., the amount ofbubbles was increased radially outwardly toward the surface portion.Further, as illustrated in FIG. 19, a multiplicity of pores 24 having adiameter of about 2 to 3 mm were present in the boundary between theopaque portion 23 and the transparent portion 22.

COMPARATIVE EXAMPLE 5

[0142] By the same procedures as employed in Comparative Example 4, anopaque ring-form silica glass article having an opaque portion and atransparent portion was made wherein the powdery silicon nitride was notused as the raw material for forming the opaque portion with all otherconditions remaining the same. The powdery quartz charged within themold as the raw material for forming the opaque portion had an apparentdensity of 1.4 g/cm³.

[0143] As illustrated in FIG. 19, the X ray diffraction analysis of theopaque ring-form silica glass article revealed that the opaque portion23 and the transparent portion 22 were in glass state. However, theopaque portion 23 has a low apparent density, i.e., 1.5 g/cm³, and whenthe glass article was cut and the cross-section was visually examined,it was found that i.e., the amount of bubbles was increased radiallyoutwardly toward the surface portion. Further, as illustrated in FIG.19, a multiplicity of pores 24 having a diameter of about 2 to 3 mm werepresent in the boundary between the opaque portion 23 and thetransparent portion 22.

[0144] The advantages of the opaque silica glass article of theinvention and the process for producing the same of the invention aresummarized as follows.

[0145] (1) The opaque portion of the opaque silica glass article iscomposed of powdery silica having uniformly dispersed therein apredetermined amount of a powdery silicon nitride. The amount anddiameter of bubbles, and apparent density of the silica glass articleare controlled by the amount of the powdery silicon nitride, theparticle diameter of the powdery silica and the melting temperature, andthus, the opaque silica glass article exhibiting excellent heatinsulating property can be obtained.

[0146] (2) The bubbles are formed in the molten material byvitrification of powdery silica and decomposition of powdery siliconnitride, and thus, impurities such as alkali metals are not incorporatedin the glass article. Further, when the raw material is melted, ahydroxyl group is not entrapped therein, but is volatilized therefrom.Therefore, the content of a hydroxyl group is minimized and theundesirable reduction of viscosity at a high temperature of the silicaglass article can be avoided.

[0147] Further, even when a transparent shaped silica glass article isused as a raw material for forming the transparent portion of the opaquesilica glass article, the resulting opaque silica glass article has agood resistance to distortion.

[0148] (3) Bubbles are not formed or formed only to a negligible extentat the boundary between the opaque portion and the transparent portion,and therefore, these two portions are firmly bonded together. When thesilica glass article is cleaned, the surface portion is not readily cutout. The glass article has a smooth surface, and the surface exhibits agood seal ability. Therefore, the silica glass article is especiallyuseful as a flange member attached to a furnace tube for heating wafers.

[0149] (4) The opaque silica glass article can be made by aheat-resistant mold of any desired shape, and thus, it can be of adesired shape such as flange-shape, ring-form, column, square pillar andhollow square pillar, or any other complicated shape. The shaping is notcomplicated.

[0150] The distortion in the production process is very minor, and asilica glass article having the finally intended size and shape can beobtained. An after-treatment such as machine finishing can be omitted or

[0151] (5) The powdery raw material for forming the opaque portion iscapable of being melted at a relatively low temperature, and thus, whena transparent silica glass article is used as a raw material for formingthe transparent portion, the opaque silica glass article can be madewithout substantial melting of the transparent silica glass article.

What is claimed is:
 1. An opaque silica glass article comprising atransparent portion and an opaque portion, wherein the glass of theopaque portion has an apparent density of 1.70 to 2.15 g/cm³ andcontains 5×10⁴ to 5×10⁶ bubbles per cm³ of the glass, said bubbleshaving an average bubble diameter of 10 to 100 μm; and the glass of thetransparent portion has an apparent density of 2.19 to 2.21 g/cm³ andthe amount of bubbles having a diameter of at least 100 μm in thetransparent portion is not more than 1×10³ per cm³ of the glass.
 2. Theopaque silica glass article according to claim 1, wherein the lineartransparency, as measured by irradiating glass article with light havinga wavelength of300 to 900 nm and expressed as the value at a thicknessof 1 mm, of the opaque portion is not larger than 5% and that of thetransparent portion is at least 90%.
 3. The opaque silica glass articleaccording to claim 1, wherein the shape of the opaque silica glassarticle is flange-form, ring-shaped, columnar, square pillar orhollow-square pillar.
 4. A process for producing an opaque silica glassarticle as claimed in any of claims 1 to 3, which comprises the stepsof: charging a heat-resistant mold with a raw material for forming theopaque portion of the silica glass article, which is a uniform mixturecomprising a finely divided silica powder having an average particlediameter of 10 to 500 μm with 0.001 to 0.05 parts by weight, based on100 parts by weight of the silica powder, of a finely divided siliconnitride powder, and a raw material for forming the transparent portionof the silica glass article so that the two raw materials are located inthe positions corresponding to the opaque portion and the transparentportion, respectively, of the silica glass article to be produced; andheating the raw materials in vacuo at a temperature in the range of themelting temperature of the raw materials and 1,900° C. whereby the rawmaterials are vitrified.
 5. The process for producing an opaque silicaglass article according to claim 4, wherein the transparentportion-forming raw material is a finely divided silica powder having anaverage particle diameter of 10 to 500 μm.
 6. The process for producingan opaque silica glass article according to claim 4, wherein the rawmaterial for forming portion of the silica glass article is atransparent molded silica glass article.
 7. The process for producing anopaque silica glass article according to claim 6, wherein thetransparent molded silica glass article is ring-shaped.